A tooth has three layers. The outermost layer is the enamel which is the hardest and forms a protective layer for the rest of the tooth. The middle and bulk of the tooth is made up of the dentin, and the innermost layer is the pulp. The enamel and dentin are similar in composition and are roughly 85% mineral, carbonated hydroxyapatite, while the pulp contains vessels and nerves which are sensitive to pressure and temperature. In this application of drilling or contouring or conditioning the enamel and dentin, the pulp's temperature sensitivity is of concern. A rise in temperature of 5.5° Celsius can lead to permanent damage of the tooth's pulp.
Over the last 10 to 15 years, research has taken place to define laser parameters that allow the enamel and dentin of a tooth to be removed, drilled, contoured or conditioned, all being removal processes, without heating the pulp. Ideally the laser pulses should vaporize the enamel and dentin converting the mass to gas with minimal residual energy remaining in the dentin to heat the pulp.
The use of lasers in dentistry has been considered since the introduction of the laser. Dental lasers used to drill and cut were the initial applications. High energy density pulses were initially used, but these pulses could potentially damage the tooth pulp or soft tissue, so lower energy pulse configurations were explored. With lower peak power energy pulses longer pulse times were used, which affected the tooth enamel detrimentally.
Various laser wavelength interactions were explored, UV to the Far Infrared, to understand the optical coupling efficiencies. Optical coupling was found to be critical with the greatest coupling being in the 2.7-3.0 μmeter and 9.3-9.6 μm wavelength ranges. When reflectance is considered, the 9.3-9.6 μmeter range was found to couple up to 3 times better than any other wavelength range.
Having identified the most effective coupling wavelength, the time and threshold to ablate hard tissue had to be determined. Research has shown that the thermal relaxation time of hard tissue is 1 to 2 μsec with a threshold ablation energy of approximately 5 mJ (milli-Joules).
Recognizing the need for laser pulses in the 9.3 to 9.6 μmeter wavelength range with microsecond pulse widths and pulse energies of 5 to 15 mJ, DC excited TEA (transversely excited atmospheric) lasers were adopted. Since the TEA lasers have a very short pulse length, i.e., hundreds of nanoseconds, the TEA lasers were modified for long pulse operation and modified pulse shapes. Additionally a RF (Radio Frequency) CW (continuous wave) laser was studied, but its shortest pulse length was only 50 μseconds, so the pulses heated the hard tissue significantly more than the shorter pulse widths.
To date, RF excited CO2 CW lasers seeking the greatest RF to Optical efficiency typically operate at 70 to 100 Torr (or about 9,332-13,332 Pascals (Pa)) and the shortest pulse lengths produced are typically 50 μseconds. Typical gas pressure for a normal RF excited CO2 laser, used in the prior art, is 80 Torr (or about 10,665 Pa). CO2 TEA lasers operating at atmospheric pressure produce 9.3 to 9.6 μmeter pulses at hundreds of nanoseconds in pulse length. TEA lasers generally do not operate in sealed operation, do not have long operating lifetimes or high pulse repetition rates, and are expensive to package. While a “long pulse” TEA laser can be manufactured to produce the optimal CO2 laser pulsing parameters, TEA lasers are larger and more expensive than RF excited lasers and therefore are not an ideal match for a dental laser application—where size and cost are critical. None of the approaches to date, therefore, have produced a full set of optimal parameters in a commercially acceptable format for effectively working with enamel and dentin, without heating the pulp.